Mycoplasma genitalium (MG) is a cause of urethritis in men and is becoming increasingly recognized for its etiologic role in cervicitis, endometritis, pelvic inflammatory disease, tubal factor infertility, and preterm birth in women. Unfortunately, this bacterium is resistant to cell wall-targeting antibiotics and to many of the antibiotics curretly used to treat primary disease and possible serious reproductive tract disease sequelae. MG infection may persist in humans for months to years in both men and women despite the induction of an inflammatory response and specific antibodies during infection. We and others have hypothesized that this persistence is based on the ability of MG to evade the host immune response by antigenic variation in two of its surface proteins, MgpB and MgpC located in its complex and unique terminal organelle. Supporting this hypothesis, we have shown that variation in mgpB and mgpC, the adjacent genes encoding these proteins, is extensive both in vivo and evolves over time in cervical/vaginal infections. However, up to this point, we have not been able to test this hypothesis by assessing the role of antibodies induced by infection on gene variation and antigenic selection of contemporary and temporally matched patient isolates. Our recent NIH-funded treatment trial for M. genitalium has given us such an opportunity. In this completed trial, M. genitalium infected men were identified and asked to return at three week intervals for three to four visits at which time sera was collected and their M. genitalium strains were cultured and characterized. These experiments are unprecedented because recent clinical isolates, not laboratory-adapted strains will be used in tour study. Thus we propose to 1) characterize the evolution of mgpB sequence variation of at different time points throughout the longitudinal study, 2) correlate the clearance of specific variable sequences in mgpB with the development of antibodies to these sequences in the infected men, and 3) determine if the antibodies to these variant sequences enhance complement-mediated killing of M. genitalium. This study is innovative in its economical use of an extremely valuable and well-characterized set of patient specimens to assess the role of gene variation on persistence of this newly recognized pathogen. This study is significant in that it will reveal, in part, the mechanisms of immune evasion in this newly recognized genital pathogen. The potential impact of our focus on the immunopathogenesis of this understudied bacterium is great in that novel targets for intervention and treatment may be identified.